The present invention relates to an optical system of an optical head employed, for example, in an optical disc drive that is capable of recording/reproducing data on a plurality of kinds of optical discs formed with cover-layers having different thickness.
Conventionally, two types of the optical systems for an optical head are known. One is known as an infinite conjugate system in which a laser beam emitted from a laser diode is collimated by collimating lens, and then converged by an objective lens. The other is known as a finite conjugate system in which a diverging laser beam emitted from a laser diode is directly converged by an objective lens.
In the latter system (i.e., finite system), the number of elements can be reduced, and therefore the optical system can be downsized. Accordingly, such an optical system has been widely employed as an optical system for CD (compact disc) drive. Generally, an image magnification of such an optical system for the CD drive is designed within a range from xe2x88x920.25 to xe2x88x920.17.
In the fnite system, on- and off-axial aberrations due to focusing and/or tracking movement of an objective lens can be sufficiently suppressed with respect to a single predetermined optical disc. However, for a plurality of discs (e.g., CD and DVD (digital versatile disc)) respectively provided with cover layers having different thickness, the aberrations cannot be suppressed sufficiently. In particular, if an optical system is designed to have the above-indicated image magnification, and the optical system is used for a DVD, the aberration is significantly greater, which prevents recording/reproducing of data on the DVD.
It is therefore an object of the invention to provide an improved optical system of a focal type which can be employed in an optical disc drive for recording/reproducing data on a plurality of optical discs respectively provided with a plurality of cover layers having different thickness.
For the above object, according to the present invention, there is provided with an optical system for an optical head employed in an optical disc drive, the optical disc drive using a plurality of optical discs, cover layers of which have different thickness, provided with a light source unit provided with a plurality of light emitting points that are arranged close to each other, a plurality of diverging light beams having different wavelengths being emitted from the plurality of light emitting points, respectively, one of the plurality of light emitting points being used depending on a thickness of a cover layer of an optical disc, an objective lens that converges each of the plurality of diverging light beams emitted from the plurality of light emitting points on a data recording surface of an optical disc, and a light receiving unit that receives a light beam reflected by the data recording surface of the optical disc and generates electronic signals corresponding to the received beam. In this optical system, an image magnification M of the objective lens satisfies a condition:
xe2x88x920.144 less than M less than xe2x88x920.099.
If the image magnification M of the objective lens satisfies the above condition, for anyone of optical discs having different cover layers and the wavelength of the selective light beam, wavefront aberration, which varies as the objective lens is moved in the axial direction, can be suppressed to an allowable level.
In particular, a light beam having a longer wavelength is selected for an optical disc having a thicker cover layer, the objective lens being located closer to the disc, and a light beam having a shorter wavelength is selected for an optical disc having a thinner cover layer, the objective lens being located farther from the disc.
It is preferable that a distance from each of the plurality of light emitting points to a surface of the cover layer of a disc is constant regardless of the thickness of the cover layer.
Optionally, the objective lens has a characteristic such that spherical aberration varies depending on the wavelength of a beam passing therethrough.
In particular, a diffraction lens structure is formed on at least one refraction surface of the objective lens, the diffraction lens structure has a characteristic such that spherical aberration varies depending on the wavelength of a beam passing therethrough.
In this case, the diffraction lens structure may be configured such that the spherical aberration changes in a undercorrected direction when the wavelength of the light beam incident thereon increases.
Optionally, the light source unit may include two light emitting points formed on a single chip.
Further optionally, the light receiving unit is mounted on the same base plate on which the light source unit is provided.
According to another aspect of the invention, there is provided an optical system for optical head employed in an optical disc drive, the optical disc drive using a plurality of optical discs, cover layers of which have different thickness, provided with a light source that selectively emits a plurality of diverging light beams having different wavelengths, one of the plurality of light beams being emitted depending on a thickness of a cover layer of an optical disc currently loaded in the disc drive, an objective lens that converges each of the plurality of diverging light beams emitted from the plurality of light emitting points on a data recording surface of an optical disc, and a light receiving unit that receives a light beam reflected by the data recording surface of the optical disc and generates electronic signals corresponding to the received beam. In this optical system, an image magnification M of the objective lens satisfies a condition:
xe2x88x920.144 less than M less than xe2x88x920.099.
With this configuration, regardless of the thickness of the cover layer of an optical disc and the wavelength of the selected light beam therefor, wavefront aberration can be suppressed to an allowable level.